Synthesis of fluorinated graphene oxide for electrochemical applications

ABSTRACT

Doping and functionalization could significantly assist in the improvement of the electrochemical properties of graphene derivatives. Herein, we report a one-pot synthesis of fluorinated graphene oxide (FGO) from graphite. The surface morphology, functionalities and composition of the resulting FGO have been studied using various surface characterization techniques, revealing that layer-structured nanosheets with ˜1.0 at. % F were formed. The carbon bound F exhibited semi-ionic bonding characteristic and significantly increased the capacitance of FGO compared to graphene oxide (GO). Further, the FGO has been employed for the simultaneous detection of heavy metal ions Cd2+, Pb2+, Cu2+ and Hg2+ using square wave anodic stripping voltammetry; and a substantial improvement in the electrochemical sensing performance is achieved in comparison with GO.

PRIOR APPLICATION INFORMATION

The instant application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/446,114, filed Jan. 13, 2017 and ‘entitled“Facile One-pot Synthesis of Fluorinated Graphene Oxide forElectrochemical Sensing of Heavy Metal Ions”, the contents of which areincorporated herein by reference.

The instant application also claims the benefit of U.S. ProvisionalPatent Application Ser. No. 62/508,080, filed May 18, 2017 and entitled“SYNTHESIS OF FLUORINATED GRAPHENE OXIDE FOR ELECTROCHEMICALAPPLICATIONS”, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Electrochemical activities of pristine graphene and its derivatives aresubjected to edge/basal plane doping, and functionalization [1-3]. Asidefrom the prevailing basal/edge plane effect, functionalization anddoping of graphene highly influence the physicochemical andelectrochemical properties of graphene and graphene oxide (GO) [3].Hence, various graphene derivatives, particularly doped with heteroatoms(e.g., N, F, Cl, B and S), have been widely explored for metal-freeelectrocatalysis, supercapacitor, and battery applications [4-8]. Amongthe doped graphene based nanomaterials, F-doping has gained greatattention because of unique properties such as its high temperatureresistance and enhanced electrocatalytic activity [9-11]. Fluorine has ahigher electronegativity than carbon, which may result in differentbonding characteristics such as ionic, semi ionic and covalent [12]. Inaddition, the electronic structure of fluorinated graphene may bealtered significantly due to its dual characteristics: (i)electron-withdrawing nature arising from the strong electronegativity ofF; and (ii) electron donating nature from the lone-pair electrons [11].Therefore, fluoro-graphene derivatives have been widely explored for avariety of applications such as batteries [5,13], biomedical devices[14], capacitors [15], and catalyst support [11]. Typically, fluorinatedgraphene is synthesized based on direct gas fluorination and plasmafluorination [10,12,16], which, however, require tedious multiple steps,harsh experimental conditions, and high energy consumption.

In the present study, we have demonstrated a facile one-pot approach forthe synthesis of FGO from graphite. The resulting FGO was systematicallycharacterized using field-emission scanning electron microscope(FE-SEM), X-ray diffractometer (XRD), Fourier transform infrared (FTIR)spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy(XPS). Further, we have investigated the electrochemistry of FGO, and adistinct behavior was observed when FGO was used in comparison to GO forheavy metal ion stripping.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodfor synthesis of about 0.5 to about 1.5 at. % fluorinated graphene oxidecomprising:

mixing n grams of graphite with about 5*n to about 40*n ml HF in asolution of about 50*n to about 150*n ml of H₂SO₄/H₃PO₄ (10-x:x, where xis equal to 0.1 to 4) with stirring at a temperature of about 30 toabout 80° C. for a first period of time;

adding about 3*n to about 10*n g KMnO₄ to the mixture and stirring themixture at a temperature of about 40 to about 80° C. for a second periodof time;

adding the reacted mixture to a container containing about 50*n to about250*n ml of ice and about 1*n to about 10*n ml of H₂O₂;

separating solid comprising about 0.5 to about 1.5 at. % fluorinatedgraphene oxide from the mixture;

rinsing the fluorinated graphene oxide; and

drying the fluorinated graphene oxide.

The first time period may be about 1 to about 5 hours.

The second time period may be about 5 to 50 hours.

In some embodiments, the fluorinated graphene oxide is rinsed with HCl,then water, then ethanol and then diethyl ether.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. (A) & (B) SEM images of graphene oxide (GO) and fluorinatedgraphene oxide (FGO). (C) TEM image of FGO. Inset shows the SAED patternof FGO. (D) XRD patterns of graphite, GO and FGO.

FIG. 2. (A) FT-IR spectra of graphite, GO and FGO. (B) Raman spectra ofGO and FGO. (C) XPS survey scan of GO and FGO. (D) & (E) High resolutionXPS spectra of C1s of GO and FGO. (F) High resolution the XPS spectrumof F1s of FGO.

FIG. 3. (A) CVs of GO and FGO modified GCE electrode in the acetatebuffer. (B) SWASVs for 2 μM analytes each of Cd(II), Pb(II), Cu(II),Hg(II) on FGO (blue line), GO (red line) and GCE (black line). (C)SWASVs of FGO for Cd(II) sensing & inset is the calibration curve. (D)SWASVs of FGO for Pb(II) sensing & inset is the correspondingcalibration curve. (E) SWASVs of FGO for the simultaneous sensing ofCd(II), Pb(II), Cu(II), Hg(II). (F) Calibration plots for theelectrochemical detection of Cd(II), Pb(II), Cu(II) and Hg(II) ions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications mentioned hereunderare incorporated herein by reference.

As used herein in the specification and claims, any numerical rangerecited herein is intended to include all sub-ranges subsumed therein.

In the present description, where used or otherwise designated to applyas described above, the terms “about” means ±20% of the indicated rangeor value unless otherwise indicated.

As discussed herein, in this method fluorine (heteroatom) doping andoxidation of graphite occurs simultaneously. Controlled fluorine dopingand semi ionic nature of the C—F bond are the specialty of this process.

In other methods, reported in literature, a minimum of two steps areinvolved, specifically, oxidation of graphite then fluorination.Graphite is oxidized using a known method such as modified hummersmethod then fluorination performed by hydrothermal [17] and directheating in F₂ atmosphere [16,18]. Fluorinating agents used are XeF₂, F₂,HF and Hexafluorophosphoric acid. Alternatively fluorinated grapheneoxide also produced from fluorinated graphite via chemical treatment[19] and mechanical exfoliation [20]. Here fluorination of graphite isfirst step then exfoliation and oxidation is second step.

In the method described herein, as the fluorinating agent is added atthe beginning of oxidation process. Fluorinating agent facilitates theoxidation of graphite and introducing fluorine on graphene sheets. Inone step oxidation as well as doping of fluorine accomplished in thismethod.

Prior art methods reported for preparation of fluorinated graphene oxideare employed harsh experimental conditions and require multiples steps.In order to simplify the method, we have developed in-situ doping ofgraphene oxide by adding the fluorinating agent to the oxidationmixture. The results have shown that we successfully incorporated ˜1.2at. % fluorine content in graphene oxide which is semi-ionically bondedwith carbon. This allows for simultaneous oxidation andfunctionalization of graphite using wet chemical synthesis.

Doping and functionalization could significantly assist in theimprovement of the electrochemical properties of graphene derivatives.Herein, we report a one-pot synthesis of fluorinated graphene oxide(FGO) from graphite. The surface morphology, functionalities andcomposition of the resulting FGO have been studied using various surfacecharacterization techniques, revealing that layer-structured nanosheetswith ˜1.0 at. % F were formed. The carbon bound F exhibited semi-ionicbonding characteristic and significantly increased the capacitance ofFGO compared to graphene oxide (GO). Further, the FGO has been employedfor the simultaneous detection of heavy metal ions Cd²⁺, Pb²⁺, Cu²⁺ andHg²⁺ using square wave anodic stripping voltammetry; and a substantialimprovement in the electrochemical sensing performance is achieved incomparison with GO.

As discussed herein, we have demonstrated a facile one-pot synthesismethod for the preparation of GO and FGO. Their compositions,morphology, and structure were investigated, revealing that FGOpossessed a higher amount and different proportions of functional groupsthan GO. The presence of fluorine was confirmed by XPS, and Ramanspectra analysis; and the fluorine content was estimated as ˜1.0 at. %.After the electrochemical reduction, FGO exhibited a much higherspecific capacitance than GO. For the first time, heavy metal ionstripping was demonstrated on metal-free FGO with a high sensitivity.The novel one-pot synthesis of the fluorine doped graphene oxidedescribed in this study opens the door to develop various halogenatedgraphene derivatives for energy, environmental and electrochemicalsensing applications.

As will be known to one of skill in the art, a high amount offluorination causes wettability and conductivity issues. In contrast, asdiscussed herein, a fluorine content at ˜1.0 at. % improves thestructural and electrochemical properties of FGO compared to GO. Ingeneral, highly fluorinated graphene exhibits amphiphobic and insulatingproperties, which is not good for electrochemical applications. Hence, afew percentages of fluorine doped graphene is useful for tunableelectrochemical properties. For example, as discussed herein, the FGOcan be used as a sensor, for energy storage, for catalyst support or forother uses which will be apparent to one of skill in the art.

For example, a sensor study demonstrated the applicability of thepresent system for heavy metal ion detection.

Using the method described herein, we obtained up to 1.5% fluorinecontent in the synthesised FGO. As discussed herein, this ˜0.5-1.5 at. %FGO is useful in various electrochemical applications such as energystorage, energy conversion, and sensor applications.

According to an aspect of the invention, there is provided a method forsynthesis of about 0.5 to about 1.5 at. % fluorinated graphene oxidecomprising:

mixing about n grams of graphite with about 5*n to about 40*n ml HF in asolution of about 70*n to about 150*n ml of H₂SO₄/H₃PO₄ (10-x:x, wherex=0.1 to 4) with stirring at a temperature of about 30 to about 80° C.for a first period of time;

adding about 3*n to about 10*n g KMnO₄ to the mixture and stirring themixture at a temperature of about 40 to about 80° C. for a second periodof time;

adding the reacted mixture to a container containing about 50*n to about250*n ml of ice and about 1*n to about 10*n ml of H₂O₂;

separating solid comprising about 0.5 to about 1.5 at. % fluorinatedgraphene oxide from the mixture;

rinsing the fluorinated graphene oxide; and

drying the fluorinated graphene oxide.

The first time period may be about 1 to 5 hours.

The second time period may be about 5 to 50 hours.

In some embodiments, the fluorinated graphene oxide is rinsed with HCl,then water, then ethanol and then diethyl ether.

As discussed herein, the FGO of the invention can be used as a sensor,for energy storage, for catalyst support or for other uses which will beapparent to one of skill in the art.

FIGS. 1A & B display the SEM images, where similar wrinkled layeredstructure was observed for both GO and FGO, indicating that the additionof HF in the improved method did not affect the overall oxidation andexfoliation of graphite. The TEM image of FGO (FIG. 1C) also confirmedthe intrinsic folds and transparent layered structure of the exfoliatedsheets. The selected area electron diffraction (SAED) pattern of the FGO(Inset of FIG. 1C) exhibited a weak hexagonal structure that wasindicative of severe exfoliation. Further, the XRD pattern of FGOcompared with GO and the graphite is illustrated in FIG. 1D. Graphiteshowed a characteristic (002) peak at ˜26.42° corresponding tointerlayer spacing (d) of 0.337 nm and crystallite size (Lc) of 20.83nm. In contrast, the graphitic peak disappeared; instead thecharacteristic 2θ peaks at 10.53° and 10.81° were observed for the FGOand GO, respectively, showing that graphite was completely oxidized.Further, the interlayer distance was calculated from the 2θ peak to be0.839, and 0.818 nm for FGO and GO, respectively; The slightly largerinterlayer distance may be attributed to the C—F formation and itsrepulsive effect [18].

FIG. 2A presents the FTIR spectra of FGO and GO, where severalcharacteristic peaks for various functional groups such as C—O group(1060 cm⁻¹), carbonyl (C═O, 1730 cm⁻¹), C═C (1620 cm⁻¹), epoxy (C—O—C,1228 cm⁻¹) were observed for both GO and FGO. However, FGO had anadditional peak at 1083 cm⁻¹, which can be attributed to the semi-ionicC—F bond [15,21]. In addition, the ratio of the C═O bond to the C═C bondwas slightly decreased due to the formation of the C—F functional groupin the FGO. Furthermore, as shown in FIG. 2B, although the Raman spectraof FGO and GO were similar, the G band was shifted from 1582 cm⁻¹ for GOto 1599 cm⁻¹ for FGO, indicating that it possessed fewer layers than theGO, which is consistent with the SEM images displayed in FIGS. 1A and1B. Additionally, the I_(D)/I_(G) ratio of FGO was calculated to be0.84, which was higher than that of GO (0.82), indicating thatfluorination caused more defects. Also, the D′ peak appeared in FGO,which was not present in GO, further confirming fluorination. All theaforementioned observations indicated that the addition of HF in theoxidative treatment could effectively exfoliate the graphene layers andgenerated a high degree of disorder in the resulting FGO. XPS was alsoemployed to characterize the composition and types of functional groupsin FGO and GO. As seen in the enlarged portion of the XPS survey scans(FIG. 2C), the existence of fluorine was confirmed by the appearance ofthe F peak at ˜686.37 eV. The composition of FGO was calculated to be65.20% C, 33.64% O, and 1.16% F; whereas the composition of GO wasestimated to be 65.17% C and 34.83% O. FIG. 2D presents the highresolution C1s spectrum of GO, showing two major peaks, which werede-convoluted into five peaks: CαC (284.5 eV), C—C (285.5 eV), C—O(286.7 eV), C═O (287.6 eV), and O—C═C (288.7 eV). As shown in the C1sspectrum of FGO (FIG. 2E), notable changes were observed in terms ofpeak intensities when compared with the C1s spectrum of GO. The C═O andO—C═O content were diminished in FGO, while the C—O content wasincreased, due to the F attack of the C═O group and the formation of C—Oand C—F bond. This was further confirmed by the high resolution F1sspectrum of FGO (FIG. 2F), where the main peak (686.25 eV) wasattributed to the semi-ionic nature C—F bond and the shoulder peakcentred at 689.62 eV corresponded to the covalent C—F bond [12].

Functionalities of a carbon surface may assist the heavy metal ionadsorption properties [22]. To improve their conductivity, FGO and GOwere electrochemically reduced at −1.2 V for 300 s in a 0.1 M acetatebuffer (pH=5.0). FIG. 3A presents the CVs of the GCE (substrate), andthe electrochemical pre-treated FGO and GO recorded in the acetatebuffer, showing a dramatic difference in the capacitance of the threeelectrodes, which may be estimated using the following equation [23]:C=A/(2×ΔE×v×m)where A—the integrated area of CV; ΔE—the potential window; v—the scanrate; and m—the mass of the GO or FGO. After the pretreatment, thecapacitance of FGO was calculated to be 94.22 F g⁻¹, which was muchhigher than that of GO (32.75 F g⁻¹).

Simultaneous electrochemical sensing of heavy metal ions was carried outas displayed in FIG. 3B, where 2.0 μM of each metal ion was used forcomparison. No notable stripping currents were seen for Cd and Pb at theGCE; and overlapped broad peaks were observed at GO, indicating that thestripping may not be easily achieved, due to the strong bonding of metalions to the GO surface [24]. In contrast, the FGO showed significantcurrents for all the four heavy metal ions with distinguishable peaks.

Thus, the low fluorine-content FGO described herein exhibitedapplicability towards simultaneous heavy metal ion sensing. It wasnoticed that Pb stripping had doublet peak, which may be due to thecomplexing mechanism with other metal ions [25,26], and that Cd had thesmallest current response compared to other three metal ions. Hence,individual detection of Cd and Pb was carried out to confirm thesensitivity and complexing mechanism, as displayed in FIGS. 3C and 3D.For Cd, a linear current response range was obtained from 0.6 μM to 5.0μM (R²=0.9987) with a high sensitivity of 4.06 μA μM⁻¹ and thecalculated lowest detection limit (LOD) of 10 nM. In the case of Pb, asingle peak was observed, confirming that the double peak was due to thecomplex formation in presence of other metal ions. A linear currentresponse range was attained from 0.3 μM to 5.0 μM (R²=0.9922) with avery high sensitivity of 10.32 μA μM⁻¹ and the LOD of 10 nM.

Simultaneous sensing of four metal ions on FGO was performed withconcentrations varying from 1.0 to 6.0 μM, and the SWASV curves andcorresponding calibration plots are presented in FIGS. 3E and 3F,respectively. All the four metals were stripped with appropriatepotential intervals and the stripping currents increased proportionallywith the increase of their concentration. The correlation factors (R²)obtained from the calibration plots were 0.9881, 0.9951, 0.9879 and0.9922 for Cd, Pb, Cu and Hg, respectively. Sensitivity of thesimultaneous detection of each metal ion was determined to be 3.64,6.05, 3.64 and 4.24 μA μM⁻¹ for Cd, Pb, Cu and Hg, respectively.

Materials and Methods

High purity graphite powder (Albany graphite deposit) was provided byZenyatta Ventures Ltd. Sulfuric acid (98%), hydrofluoric acid (50%),copper(II) nitrate trihydrate (99.0%), mercury(II) nitrate monohydrate(≥98.5%), and lead(II) nitrate (≥99.0%) were sourced from Sigma Aldrich.Analytical grade reagents (phosphoric acid (85%), potassium permanganate(≥99.0%), potassium chloride (99.0), acetic acid (≥99.7%), sodiumacetate (≥99.0%) and cadmium(II) nitrate tetra hydrate (98%)) were usedas received without further purification. Pure water (18.2 MΩ cm,Nanopure® diamond™ UV water purification system) was used for aqueoussolution preparation.

FGO was synthesized by the improved Hummers' method with somemodifications [27]. Briefly, 1 g of graphite was added in the mixture of90 ml H₂SO₄, 10 ml H₃PO₄, and 20 ml HF. After vigorous stirring at 50°C. for two hours, 4.5 g of KMnO₄ was added slowly into the reactionmixture and was stirred continuously for another 15 hours. Then 100 mlice was added to the reaction mixture followed by an addition of 5 ml of30% H₂O₂. The resulting FGO was separated and rinsed with 30% HCl, purewater, ethanol, and diethyl ether. Finally, the resulting yellowishbrown solid was dried in the oven at 50° C. For comparison, GO was alsoprepared using the same procedure, but without the addition of HF.

Morphological studies and surface characterization were conducted usingFE-SEM (Hitachi SU-70), TEM (JOEL 2010), XRD (Panalytical Instrument),FTIR spectrometer (Thermo scientific), Raman spectroscopy, XPS (Thermoscientific). Cyclic voltammetry (CV) and square wave voltammetry (SWV)were conducted using a CHI 660E electrochemical workstation. A 2.5 mgsample of FGO or GO was dispersed in 1 ml of isopropanol-water (1:1)mixture using ultrasonication for 30 minutes. Then a 3 μL aliquot of theFGO or GO dispersed solution was drop-cast on a polished glassy carbonelectrode (GCE) surface and dried. The FGO/GCE and GO/GCE werepretreated electrochemically at a constant potential −1.2 V vs Ag/AgClin a 0.1 M acetate buffer solution (pH 5.0). Two steps were involved inthe heavy metal ion sensing: (i) deposition of metal ions at −0.9 V for175 s; and (ii) square wave anodic stripping voltammetry (SWASV)conducted from −0.9 to 0.6 V.

The scope of the claims should not be limited by the preferredembodiments set forth in the examples but should be given the broadestinterpretation consistent with the description as a whole.

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The invention claimed is:
 1. A method for synthesis of about 0.5 to about 1.5 at. % fluorinated grapheme oxide comprising: mixing about n grams of graphite with about 5*n to about 40*n ml HF in a solution of about 50*n to about 150*n ml of H₂SO₄/H₃PO₄ at a ratio of 10-x:x, where x=0.1 to 4 with stirring at a temperature of about 30 to about 80° C. for a first period of time to generate a first mixture; adding about 3*n to about 10*n g KMnO₄ to the first mixture to generate a second mixture and stirring the second mixture at a temperature of about 40 to about 80° C. for a second period of time; adding the second mixture to a container containing about 50*n to about 250*n ml of ice and about 1*n to about 10*n ml of H₂O₂ to generate a third mixture; separating solid comprising about 0.5 to about 1.5 at. % fluorinated grapheme oxide from the third mixture; rinsing the fluorinated graphene oxide; and drying the fluorinated graphene oxide.
 2. The method according to claim 1 wherein the first time period is about 1 to 5 hours.
 3. The method according to claim 1 wherein the second time period is about 5 to about 50 hours.
 4. A method for synthesis of about 0.5 to about 1.5 at. % fluorinated graphene oxide comprising: mixing about n grams of graphite with about 5*n to about 40*n ml HF in a solution of about 50*n to about 150*n ml of H₂SO₄/H₃PO₄ at a ratio of 10-x:x, where x=0.1 to 4 with stirring at a temperature of about 30 to about 80° C. for a first period of time to generate a first mixture; adding about 3*n to about 10*n g KMnO₄ to the first mixture to generate a second mixture and stirring the second mixture at a temperature of about 40 to about 80° C. for a second period of time; adding about 50*n to about 250*n ml of ice to the second mixture, followed by adding about 1*n to about 10*n ml of H₂O₂ to the second mixture to generate a third mixture; separating solid comprising about 0.5 to about 1.5 at. % fluorinated graphene oxide from the third mixture; rinsing the fluorinated graphene oxide; and drying the fluorinated graphene oxide.
 5. The method according to claim 4 wherein the first time period is about 1 to about 5 hours.
 6. The method according to claim 4 wherein the second time period is about 5 to about 50 hours. 